Transportation systems, such as an aircraft, a ship, or a train, typically provide only a limited supply of power. This power supply serves not only those systems that are essential, but non-essential equipment as well. Aboard an aircraft, for example, the propulsion system provides a finite amount of power to operate both essential equipment, such as life-support, communication, and flight control, and non-essential equipment, such as coffee makers, in-flight commercial phones, in-seat entertainment centers, and a variety of devices operated by passengers.
Because power is limited, non-essential equipment must compete with other non-essential equipment for power. If the load from non-essential equipment exceeds the allowable load, some essential equipment may be deprived of power. Furthermore, the power supply itself may be damaged from the additional loads.
Several systems and methods have been developed to monitor and adjust the power requirements of the load, determining which equipment to turn off and/or causing the equipment to enter a power saving mode (“load-shedding”).
U.S. Pat. Nos. 5,754,445 and 6,046,513, the disclosures of which are incorporated by reference herein in their entirety, describe a power management system in which the load at consumers' outlets (where music players, computing devices, etc. may be plugged in) is continually monitored, and certain not-in-use outlets are disabled when the system enters a power-management mode. A power management circuit is connected to various decentralized power control units (also referred to as power supply units or power converters) each supplying power to one or more outlets. A signal on a line connected to the power control units (hereafter called an ENABLE keyline) indicates whether the system is in an enabled mode or in a power-managed mode. The state of this signal enables or disables outlets that are not in use. In these systems, a consumer device that is in use is not turned off when entering power-management mode.
A system as described in the above-referenced patents is shown schematically in FIG. 1. Power management circuit 1 monitors the power consumed by the system 3 and compares this level, using a limit comparator 5, to the power load limit 6. The ENABLE signal is transmitted to the power control units 2 on ENABLE keyline 7; a SET condition signifies that system power is available at the various outlets. When the limit comparator 5 determines that the total power sensed is below the limit, the ENABLE keyline 7 is SET. When a consumer connects a device to an outlet 11, a request 9 for power is initiated to the control unit. If the ENABLE keyline is SET when the request for power is initiated, the output control 8 enables power to the device plugged into the outlet 11.
If the limit comparator 5 shows that the power limit has been reached, the ENABLE keyline 7 is reset to signify the additional power is not available. Accordingly, unused power outlets are disabled until the total system power consumption falls below a second threshold (as determined by limit comparator 5), at which time those outlets are re-enabled for use by the consumer. Should a consumer have plugged in a device while the system was in the power management mode (so that the device was plugged into a disabled outlet), control latch 10 requires that the user unplug the device and plug it in again to initiate a request for power. In addition, an indicator is typically provided on the outlet to show that power is available. If the indicator is extinguished, the indicator is locked off, so that there will not be an indication that power has been restored until the device is unplugged. This is inconvenient for the consumer, since it is difficult for the consumer to know whether power is available or has been restored in these situations.
Systems have been devised to avoid this consumer inconvenience by automatically connecting power to devices plugged in during a power management phase. In one such system, each power control unit is provided with timers, with a timer connected to each outlet. The timers can impose a delay between activation of the individual outlets in the power control unit. If the ENABLE keyline is SET when a request for power is initiated, the output control enables power to the consumer device plugged into the outlet; no delay is initiated as long as power is available when the device is plugged in. Should the consumer have plugged in the device while the system is in the power management phase (that is, the ENABLE keyline signifies that no additional power is available and unused outlets are disabled), the timer limits when the outlet may be reconnected. The timer starts a delay period during which the limit comparator senses the total power consumption. When the delay period expires, if additional power is still available the outlet is automatically activated. The next outlet in the power control unit is likewise activated with a delay if a device was plugged in during the power management phase; this continues until the final outlet in the unit is activated.
More recently, a system has been disclosed having specifically random timer action for enabling of individual outlets connected to a decentralized power supply unit. This system is shown schematically in FIG. 2. Power management circuit 42 monitors the power consumed by the system 44 and compares this power level to the power limit 43 using limit comparator 41. When the limit comparator 41 determines that the total power sensed is below the limit, the ENABLE keyline 33 is SET. As a consumer connects a device to outlet 39, a request for power 38 is initiated to the power supply unit 45. If the ENABLE keyline is SET when the request for power is initiated, output control 40 enables power to the device plugged into outlet 39. No delay is initiated as long as power is available when the device is plugged in.
If the limit comparator 41 determines that the power limit has been reached, the ENABLE keyline 33 is reset to signify the additional power is not available. Unused power outlets 46, 47 accordingly are disabled until the total system power consumption falls below a second threshold as determined by limit comparator 41 and re-enables outlets 46, 47 for use by the consumer.
Should a consumer have plugged in while the system is in the power management mode (that is, ENABLE keyline 33 signifies that no additional power is available and unused outlets are disabled), and subsequently the power consumption 44 falls below the threshold determined by limit comparator 41, then the ENABLE keyline is SET and timers 35, 36, 37 determine when each respective outlet will be reconnected. In this system, the timers are initiated at the same time, but cause delays of random lengths at the respective outlets. As each random delay time expires, the associated outlet is enabled if the ENABLE keyline 33 remains SET.
All of the decentralized power supply units 45, 50, etc. receive the ENABLE keyline signal at the same time. All of the random timers in each power supply unit thus start at the same time. Since the delays are of random lengths, the risk of two loads being activated simultaneously is reduced. However, in a system where multiple power supply units are attached, a number of outlets with loads still may be activated within close proximity in time. This may not allow enough time for the power management circuit 42 to measure the consumed power and control the ENABLE keyline to limit the number of outlets activated to avoid a system overload.
In the timed automatic-connect system and random-timer system described above, there may be loss of control of the power load during the re-connect sequence so that the system maximum is inadvertently exceeded. For example, laptop computers typically require several seconds before the charging circuit reaches full current. In addition to the device delay time, there is a period of time required for the power management circuit to measure the power consumed, and for the limit comparator to determine if additional power is available and set the ENABLE keyline appropriately. If the system has several power control units, a number of outlets with loads may be activated simultaneously or nearly simultaneously, and thus cause overload of the power system.
The systems described just above avoid inconvenience to the consumer, but allow a potential for loss of system control and overload of the power system. There remains a need for a load management system for limited power environments that can re-connect devices while maintaining control of the overall load.